Yes, acres will be down, but according to long term wheat yields and EL NINO, wheat yields especially HRW will tend to be above trend,
 which would mitigate any decrease in acreage.


 "At the national level El Nino and La Nino conditions both resulted in above-trend yields an equal number of times, or 45% of the time. Furthermore, over the 64 years of data above-trend yields overall were experienced 42% of the time. This is to say that across that across the country, El Nino events have historically resulted in above- (and below-) trend yields at nearly the same frequency as average. The same is true for La Nina."

Figure 1. Departure of Trend-Adjusted Wheat Yields from Average (1951-2014) and ONI Conditions for the Fall (Warm, El Nino favoring in red; Cool, La Nina favoring in blue; Neutral in black). Data Sources: USDA NASS, National Weather Service’s Climate Prediction Center.

Impacts Vary

"While above trend-yields do not occur more frequently during fall El Nino conditions at the national level, there is substantial variation at the state-level. Shown in figure 2 is the share of historic fall El Nino conditions that resulted in above-trend state-level yields.

Across the Plains states, the historic probability of above-trend yields during El Nino conditions were high, 65% or greater. This was especially true in the Southern Plains states of Kansas, Oklahoma, and Texas, which accounted for 49% of the winter wheat planted in 2014. Texas, for instance, has historically experienced above-trend yields 70% of the time fall El Nino conditions were present [2].

In Kansas, the largest producer of winter wheat by planted acres (2014), years where El Nino conditions have occurred during the fall months has resulted in yields as high as nearly 12 bushel above trend to as low as 10 bushel below trend (not shown). Quite a range of potential outcomes. Overall, El Nino conditions have resulted in above trend yields 65% of the time."

Figure 2. Percent of Times Yields were Above Trend Average in El Nino Conditions, by State. Data Soucres: USDA NASS, National Weather Service’s Climate Prediction Center.